This invention relates to integrated arrays of thin film photovoltaic cells and more particularly to a method for scribing back electrical contacts on such arrays to form back electrodes for the separate cells.
Much work has been done in recent years to develop practical thin film photovoltaic cells. It is generally believed that ultimately the cost of producing such cells will be considerably less than the cost of commerically available solar cells fabricated from single crystal or polycrystalline silicon slices. Cost reduction is based on the reduction in the amount of semiconductor material required for thin film cells. In addition, it is expected that thin film processes will be more easily automated, so that labor costs can also be reduced.
U.S. Pat. No. 4,315,096 issued to Tyan, et al, on Feb. 9, 1982 describes a somewhat typical thin film photovoltaic cell array. This patent is hereby incorporated by reference. The Tyan, et al, patent in particular discusses the desirability of depositing the various layers of a thin film solar array in continuous layers which are then formed into a plurality of separate but series connected cells. Tyan teaches that the cells may be formed either by mechanically scribing the various layers at the appropriate stages of manufacture or by use of laser scribing techniques.
The various layers of materials in such thin film solar cell structures have considerably different mechanical characteristics. Thus the semiconductor layer, typically silicon, is quite hard and brittle. In contrast, the back contact layer is typically aluminum which is quite soft and malleable. These characteristics cause several problems in mechanical scribing of the back contact on a production basis. Based on the softness of the back contact it would appear to be relatively simple to remove the bulk of the material by drawing a scribe point across the desired scribe line. However, in practice, the aluminum usually smears or flows and this action makes it difficult to remove the bulk of the metal. When sufficient force is applied to remove most of the metal from the scribe line the smearing usually leaves a thin layer shorting the adjacent back contact segments together. As noted in the Tyan patent, it is generally desirable to remove at least a portion, if not all, of the semiconductor layer along the scribe path. When a sufficiently hard scribe point is applied with sufficient pressure to cut through both the back contact material and the semiconductor layer, shorting can still occur. This is typically caused by smearing of the back contact metal along the scribed edges of the semiconductor layer causing a direct short circuit between the front and back faces of the active device. In addition when pressures of this level are applied damage often occurs to the transparent conductive oxide layer under the semiconductor layer or even to the glass substrate.
Thus it is seen that it is desirable in the manufacture of thin film photovoltaic cell arrays to provide an improved method for separating a back contact layer into separate back electrodes for the individual cells which are truly electrically isolated from both adjacent back electrodes and from front electrodes.